In general, ink jet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming media. A phase change ink jet printer employs phase change inks that are substantially solid or gelatinous at ambient temperature and that transition to a liquid phase at an elevated temperature. The liquid phase change ink, also referred to herein as melted ink or molten ink, can then be ejected onto a printing media by a printhead onto an image receiving substrate, referred to as direct to media printing, or onto an intermediate imaging member and subsequently transferred to an image receiving substrate, referred to as indirect printing. Once the ejected ink is on the image receiving substrate, the ink droplets quickly solidify to form an image.
In both the direct and offset printing architecture, the image receiving substrate may comprise individual media sheets or a substantially continuous supply of media, also referred to as a media web. In a web printer, the continuous supply of media is typically provided in a media roll mounted onto rollers that are driven by motors. A loose end of the media web is passed through a print zone that includes a plurality of printheads arranged to deposit the molten phase change ink onto the web to form images. Beyond the print zone, the media web is gripped and pulled by mechanical structures so a portion of the media web continuously moves through the print zone. A high pressure roller nip, also referred to as a spreader, arranged downstream from the print zone may be used after the ink is jetted onto the web in the print zone to spread the ink on the web to achieve the desired print quality. The function of the spreader is to take what are essentially isolated droplets of ink on web and smear them out to make a continuous layer by pressure and/or heat so that spaces between adjacent drops are filled and image solids become uniform.
In order to achieve acceptable ink spreading performance at the spreader, as well as other image quality metrics, such as ink color mixing, ink to web adhesion, and the like, current phase change ink print processes require that the web temperature be maintained at a target temperature within the print zone. The target temperature is dependent upon a number of factors, such as the media type and ink formulation. For example, for a nominal 75 gsm paper, phase change ink print processes may require that the web be heated to and maintained at a temperature of approximately 55° C. in the print zone. To achieve the target preheating temperature, previously known systems typically included a preheater in the form of a heated roller positioned to be partially wrapped by the web prior to the web entering the print zone. The preheat roller in such previously known systems is heated to a temperature that enables conductive heat transfer to occur between the web and the roller surface to bring the temperature of the web to the target preheating temperature. In addition, heaters in the form of rollers, backing members, or the like may be arranged in the print zone to maintain the web of media at the target temperature as the ink is deposited thereon by the printheads.
One challenge faced in preheating the web to the target temperature and maintaining the web at the target temperature through the print zone is shrinkage of the media. For example, under common ambient atmospheric conditions, e.g., approximately 25° C., paper commonly used for ink jet printing can have a moisture content that may range, depending on actual humidity, from about 1% to 10%. When a continuous web of paper is brought into contact with a preheat roller, the moisture in the fibers of the paper is driven out and the paper begins to shrink. As mentioned, some previously known systems have a target temperature for the preheater of about 55° C. While the preheat roller in such systems may be capable of heating the web to the desired target temperature, the web may not be heated long enough prior to entering the print zone for the paper's water content to equilibrate. Thus, even when preheated to the target temperature, the web may continue to shrink after entering the print zone which makes registering colors more difficult. Tests have shown that one 20″ wide web of paper heated to 55° C. and kept at that temperature may shrink by as much as 2 mm during printing.
Another challenge faced in operating a web preheater is maintenance of a consistent, or uniform, temperature at the heating surface of the preheater that enables the web to be heated to the target temperature. As mentioned, the web is typically at ambient temperature prior to contacting the preheat roller. Therefore, the temperature of the web may have to be raised approximately 30° C. to reach a target preheating temperature of 55° C. The surface of the preheat roller loses energy, or heat, as it is contacted by the lower temperature web. Consequently, a preheat roller may have to be heated to a temperature well above the target preheat temperature in order to compensate for the loss of heat that results from contact with the web. As an example, to achieve a target preheating temperature of approximately 55° C. at a web speed of approximately 80 ips, the preheat roller may have to be heated to about 70-75° C. The large temperature gradient of the web as it is heated from ambient to about 55° C. by the heated roller surface may cause the web to remove more energy from the roller surface than the heating element of the preheat roller can replace in a timely fashion. Tests have shown that a preheat roller heated to about 75° C. and contacted with a web traveling about 80 ips may have a drop in temperature of as much as 4-5° C. As a result of the temperature drop, the temperature at the surface of the preheat roller may be subject to temperature fluctuations which in turn may cause uneven heating of the web and inconsistent image quality. Temperature fluctuations and variations at the surface of the preheat roller may also cause diameter changes along the axis of the roller that may adversely impact the ability of the imaging device to register images on the web formed by the different printheads.
To address the challenges of web shrinkage and preheat roller temperature fluctuations, previously known systems lowered the target temperature for the preheat roller to, for example, 45° C., thus decreasing the difference between the incoming web temperature and the preheat roller set point temperature, in this case, from 30° C. to approximately 20° C. Decreasing the temperature difference between the incoming web and the pre-heat roll set point results in the preheat roller losing less energy to heat the web to the target temperature thereby reducing the magnitude of temperature variations in the preheat roller and the problems associated therewith. For example, at a lower preheat temperature set-point of 45° C., the preheat roller surface temperature drops very little, e.g., approximately 1° C., when contacted by the web at ambient temperature, and the lower preheat and print temperatures also result in less moisture being driven from the media so that there is a smaller change in media size during printing. While lowering the preheat target temperature of the preheat roller may be effective in reducing the problems associated with temperature fluctuations at the roller surface and media shrinkage in the print zone, the lower web temperature may decrease the image quality of the resulting images due to reduced spreading performance at the spreader and reduced ink to web adhesion.